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<head>
  <meta http-equiv="Content-Type" content="text/html; charset=utf-8">
  <title>无标题文档</title>
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<body>
  <p id="intro">我的课程</p>
  <ul>
    <li title="JS">JavaScript</li>
    <li title="JQ">JQuery</li>
    <li title="">HTML/CSS</li>
    <li title="JAVA">JAVA</li>
    <li title="">PHP</li>
  </ul>
  <h1>以下为li列表title的值,当title为空时，新设置值为"WEB前端技术":</h1>
  <script type="text/javascript">
    var Lists = document.getElementsByTagName("li");
    for (var i = 0; i < Lists.length; i++) {
      var text = Lists[i].getAttribute("title");
      document.write(text + "<br>");
      if (text == "") {
        Lists[i].setAttribute("title", "WEB前端技术");
        document.write(Lists[i].getAttribute("title") + "<br>");
      }
    }
  </script>
</body>

</html>